The basic principal of the Stirling external combustion engine is simple, being no more than the tendency of a contained gas working fluid to expand or increase in pressure when heat energy is added and to contract or decrease in pressure as energy is removed from the contained gas working fluid. Useful power is derived from the difference of heat input and heat rejection less frictional, inertial, and other losses. Stirling engine structure which uses a regenerative heat exchange system is a heat pump operated in reverse. Any available heat source can be applied to the Stirling heat pump and converted into useful mechanical energy.
Previous investigators in Stirling machine research proposed the concept of balanced compounding in 1978. The Stirling cycle is one example of a general class of regenerative cycle external heat source engines. The principal of the balanced compound machine would integrate four alpha subsystems into parallel opposed cylinder geometry consisting of four double acting opposed reciprocating pistons in two pairs such that each piston pair controls two expansion spaces phased 180 degrees apart. It is important to note here that "double acting" as referred to by Stirling literature refers to the action of the pressure on either or both sides of a piston face; whereas the term "double acting" applied to other mechanical mechanisms describes the nature of the motion of two opposed piston faces which move away from or alternately toward each other during any reciprocating stroke cycle which causes a volume change in a given cylindrical space relative to a common cylinder axis.
A brief description of the Stirling engine cycle and the three main classifications of Stirling engines based on mechanical differences is briefly outlined below. The well known Stirling cycle as related to reciprocating piston engines comprises:
by means of piston movement some of the gas is transferred back and forth between the hot space and the cold space; PA1 when more of the gas is in the hot space the pressure rises and is caused to work against a piston face; PA1 the pressure falls as expansion stroke of the power piston occurs back to the cold space of the displacer; PA1 the power piston is caused to return in a compressive stroke when some of the gas has been moved back into the cold space of the displacer and the pressure has fallen; PA1 because the pressure is lower, the force on the power piston is less on its inward compressive stroke than it was during the outward expansive movement so that a net amount of work is gained from the total cycle.